Bearing assemblies are employed to permit sliding or rotating movement between parts with a minimum of friction. For example, ball or roller bearing assemblies are used to support the drive shaft, or spindle, of a computer magnetic disk drive. In modern disk drive assemblies, the mechanical clearance between the surface of the magnetic disk which stores data and the read/write head which transfers data to and from the disk is extremely small, i.e., about 10 microinches. Any particulate contamination on the disk surface or in the region of the read/write head is likely to cause a malfunction of the assembly. Consequently, the computer disk and the head must be enclosed and must be protected against contamination from the outside environment. The motor, which drives the disk shaft, and the bearings which support the disk shaft are a source of contamination due to their mechanically movable parts. However, it is generally inconvenient to enclose the motor. Thus, a ferrofluid exclusion seal is typically employed on the drive shaft to prevent contaminants from the motor and the bearings and from the external environment from reaching the interior of the disk assembly along the drive shaft.
The trend toward miniaturization of disk drives has required thinner and thinner drives, thereby severely limiting the axial space available for magnetic fluid seals and requiring seal designs with a very small axial length, or thickness. Magnetic seals for use in small disk assemblies, such as 51/4 inch and 31/2 inch spindles, are typically required to have an axial thickness on the order of 1.3 mm, while disk shaft bearings are typically required to have an axial thickness on the order of 5 mm.
Various bearing assemblies and separate ferrofluid seals have been disclosed in the prior art to perform the exclusion function. Multiple stage seals have been employed in conjunction with ball bearings to support a shaft as disclosed in U.S. Pat. No. 3,620,584, issued Nov. 16, 1971 to Rosensweig. The separate prior art seal and bearing assemblies do not meet the seal thickness requirements of present disk drives.
It is known in the prior art to utilize a magnetic fluid as the lubricant in ball bearing assemblies. Ball bearing assemblies utilizing magnetic balls to retain a ferrofluid lubricant in place have been disclosed in U.S. Pat. No. 3,977,739, issued Aug. 31, 1976 to Moskowitz et al. The ferrofluid is retained by a magnetic field in the regions where the bearing balls contact the bearing races. As a result, the bearing runs cooler and its life is extended. However, any leakage of the ferrofluid from the bearing is a source of contamination to the sealed disk head assembly and reduces the bearing effectiveness. A bearing having a magnetic fluid as a lubricant and utilizing seals on each end is disclosed in U.S. Pat. No. 4,043,616, issued Aug. 23, 1977 to Zimmer. Annular magnets, attached to the housing or to the shaft at each end of the bearing, define gaps in which a sealing magnetic fluid is retained. The Zimmer configuration is not suitable for use in applications requiring a small axial thickness, such as the disk drive assembly described above. Each annular magnet has an axial dimension on the same order as the ball bearing. Reduction of the axial thickness of the magnets to thicknesses on the order of 1 millimeter would reduce the magnetic field below the level which is required for holding the magnetic fluid in the gaps. Furthermore, the magnets are separated by spacers from the bearing, which increases the overall axial length of the sealed bearing. A radially polarized magnet has been positioned between the races of a bearing to provide an exclusion seal in U.S. Pat. No. 4,531,846, issued July 30, 1985 to Raj. However, the Raj patent does not disclose sealing of the bearing and is not compatible with achieving a small axial thickness since a reduction in magnet thickness would reduce the magnetic field below the level required for holding the magnetic fluid.
It is a general object of the present invention to provide improved bearing assemblies.
It is another object of the present invention to provide rolling element bearing assemblies which are both sealed and lubricated with a magnetic fluid.
It is yet another object of the present invention to provide rolling element bearing assemblies which are electrically conductive.
It is still another object of the present invention to provide sealed bearing assemblies with a minimum axial dimension.
It is a further object of the present invention to provide bearing assemblies sealed with ferromagnetic fluid wherein a magnetically permeable shaft is not required.
It is a further object of the present invention to provide sealed rolling element bearing assemblies which are easily manufactured and which are low in cost.